global - migrate strncmp literals to str_begins

global - migrate strncmp literals to str_begins

Simplify a bunch of strncmp(,,number) calls.

git ls-files \*.[ch] | xargs perl -p -i -e 's/strncmp\((.*?), ?(\".*?\"), ?(\d+)\) == 0/str_begins($1, $2)/g'
git ls-files \*.[ch] | xargs perl -p -i -e 's/strncmp\((.*?), ?(\".*?\"), ?(\d+)\) != 0/!str_begins($1, $2)/g'

I ran a longer script to verify that all of the string literals and
the length matched. They didn't:

$ git grep strncmp | perl -ne 'print if(m/strncmp\([^,]*,\s*"(.*?)",\s*(\d+)/ and ($s=$1,$t=$2,$s=~s/\\[tn]/#/g,length($s)) != $t)'
src/auth/db-oauth2.c:               if (strncmp(field, "oauth2:", 8) == 0 &&

With the new functions, that kind of typo is impossible.

Signed-off-by: Phil Carmody <phil@dovecot.fi>