Add simplified VEXT loading as -E<name>

[nigoroll]

Similar to how vmods can be loaded by their base name, we now support loading
extensions using, for example, -Estd:

If a path is specified (-E argument contains "/"), it is taken literally as
before. Otherwise, the -E argument is expanded to libvmod_%s.so and a search in
vmod_path is conducted..

The first commit is just a minor refactoring to move the VEXT code to mgt.

[walid-git]

The asan job is reporting a leak here:

*** v1 debug|=================================================================
*** v1 debug|==111424==ERROR: LeakSanitizer: detected memory leaks
*** v1 debug|
*** v1 debug|Indirect leak of 32 byte(s) in 1 object(s) allocated from:
*** v1 debug| #0 0x7f6fc6355340 in calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:77
*** v1 debug| #1 0x558811bd45d8 in VFIL_setpath /workspace/lib/libvarnish/vfil.c:307
*** v1 debug| #2 0x558811a60e0a in vext_argument mgt/mgt_vext.c:104
*** v1 debug| #3 0x558811a30cf3 in main mgt/mgt_main.c:808
*** v1 debug| #4 0x7f6fc55d01c9 (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
*** v1 debug| #5 0x7f6fc55d028a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
*** v1 debug| #6 0x558811730ab4 in _start (/workspace/bin/varnishd/varnishd+0x85cab4) (BuildId: 9538b9af3300f39414978a5e327595cb917edf8c)
*** v1 debug|
*** v1 debug|Indirect leak of 32 byte(s) in 1 object(s) allocated from:
*** v1 debug| #0 0x7f6fc6355340 in calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:77
*** v1 debug| #1 0x558811bd52c1 in VFIL_setpath /workspace/lib/libvarnish/vfil.c:323
*** v1 debug| #2 0x558811a60e0a in vext_argument mgt/mgt_vext.c:104
*** v1 debug| #3 0x558811a30cf3 in main mgt/mgt_main.c:808
*** v1 debug| #4 0x7f6fc55d01c9 (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
*** v1 debug| #5 0x7f6fc55d028a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
*** v1 debug| #6 0x558811730ab4 in _start (/workspace/bin/varnishd/varnishd+0x85cab4) (BuildId: 9538b9af3300f39414978a5e327595cb917edf8c)
*** v1 debug|
*** v1 debug|Indirect leak of 13 byte(s) in 1 object(s) allocated from:
*** v1 debug| #0 0x7f6fc634f4e8 in strdup ../../../../src/libsanitizer/asan/asan_interceptors.cpp:578
*** v1 debug| #1 0x558811bd493e in VFIL_setpath /workspace/lib/libvarnish/vfil.c:312
*** v1 debug| #2 0x558811a60e0a in vext_argument mgt/mgt_vext.c:104
*** v1 debug| #3 0x558811a30cf3 in main mgt/mgt_main.c:808
*** v1 debug| #4 0x7f6fc55d01c9 (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
*** v1 debug| #5 0x7f6fc55d028a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
*** v1 debug| #6 0x558811730ab4 in _start (/workspace/bin/varnishd/varnishd+0x85cab4) (BuildId: 9538b9af3300f39414978a5e327595cb917edf8c)
*** v1 debug|
*** v1 debug|-----------------------------------------------------
*** v1 debug|Suppressions used:
*** v1 debug| count bytes template
*** v1 debug| 2 136 HSH_config
*** v1 debug| 10 480 vsmw_newcluster
*** v1 debug|-----------------------------------------------------
*** v1 debug|
*** v1 debug|SUMMARY: AddressSanitizer: 77 byte(s) leaked in 3 allocation(s).

[nigoroll]

Yes, we usually leak in mgt.