elfctl: fix operations with multiple features on multiple files

[emaste]

Summary

Previously an invocation like elfctl -e +feature1,feature2 file1 file2 would set both feature flags in file 1 but only feature1 in file2 due to strsep().

Reported by: jrm
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

[jrm]

Looks good from a testing perspective.

jrm@phe /usr/obj/usr/src/amd64.amd64/usr.bin/elfctl % sudo ./elfctl -e '+noaslr,wxneeded' /usr/local/bin/ztop /usr/local/bin/zstd
jrm@phe /usr/obj/usr/src/amd64.amd64/usr.bin/elfctl % ./elfctl /usr/local/bin/ztop /usr/local/bin/zstd | grep 'noaslr\|wxneeded'
noaslr          'Disable ASLR' is set.
wxneeded        'Requires W+X mappings' is set.
noaslr          'Disable ASLR' is set.
wxneeded        'Requires W+X mappings' is set.
jrm@phe /usr/obj/usr/src/amd64.amd64/usr.bin/elfctl % sudo ./elfctl -e '-noaslr,wxneeded' /usr/local/bin/ztop /usr/local/bin/zstd
jrm@phe /usr/obj/usr/src/amd64.amd64/usr.bin/elfctl % ./elfctl /usr/local/bin/ztop /usr/local/bin/zstd | grep 'noaslr\|wxneeded'
noaslr          'Disable ASLR' is unset.
wxneeded        'Requires W+X mappings' is unset.
noaslr          'Disable ASLR' is unset.
wxneeded        'Requires W+X mappings' is unset.

There are a few minor, pre-existing whitespace issues (space inserted next to a tab) on lines 85, 409, 448.