grep: fix grep -x to not set REG_NOSUB

When -F isn't specified (and !ENABLE_EXTRA_COMPAT), grep -x uses
regexec's regmatch_t output to determine if the match was the entire
line. However it also set the REG_NOSUB flag which makes it ignore the
regmatch_t argument.

Add an exception to the setting of REG_NOSUB for OPT_x and add some test
cases to test the behaviour of -x.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Natanael Copa <ncopa@alpinelinux.org>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Merge git://busybox.net/busybox