uip/mhbuildsbr.c: Fix long-line truncation when BUFSIZ is 1024.

uip/mhbuildsbr.c: Fix long-line truncation when BUFSIZ is 1024.

a23477eb changed one char array from BUFSIZ to NMH_BUFSIZ elements, but
the long line read into it was passed to user_content() that
strncpy(3)'d it, silently truncating, to another char array, still
BUFSIZ long.  This show up on platforms where BUFSIZ is 1024 versus
NMH_BUFSIZ's minimum of 8192.