maint: prefer memeq to memcmp

maint: prefer memeq to memcmp

* gl/modules/randread-tests (Depends-on): Add stringeq.
* gl/tests/test-rand-isaac.c (main):
* src/copy.c (source_is_dst_backup):
* src/digest.c (b64_equal):
* src/install.c (have_same_content):
* src/ls.c (main, parse_ls_color):
* src/nl.c (check_section):
* src/od.c (write_block):
* src/seq.c (seq_fast):
* src/stty.c (eq_mode):
* src/system.h (is_nul):
Prefer memeq to memcmp when either will do.