elfctl: fix operations with multiple features on multiple files

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 the string being modified by strsep()).

Reported by: jrm
Tested by: jrm
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34283

(cherry picked from commit 82b611ed18e6bfaeadf152ef9216230548c25a8a)