Make many m_getfld() buffer parameters NMH_BUFSIZ big.

Make many m_getfld() buffer parameters NMH_BUFSIZ big.

char arrays were often stdio.h's BUFSIZ large, and that's 8192 with here
with glibc 2.25-1 on Linux x86_64, so using NMH_BUFSIZ is no different
as that's max(BUFSIZ, 8192).  But some were 256 or 512 with a local
SBUFSIZ macro and they caused scan(1) to truncate a field, or pick(1) to
not spot text because the field was truncated.

Delete bad-input/test-header's test for m_getfld()'s detection for a
header without a colon that's under the header-length NAMESZ limit, but
longer than the passed in buffer.  This no longer happens in scan as the
buffer is larger than the longest allowed header.