m4: Fix eval output width

According to POSIX, the optional third argument is the minimum number
of digits to print regardless of sign. We interpreted it as the minimum
width of the output including the sign. Additionally, the variable used
to hold this value was confusingly named “maxdigits”.

PR: 293214
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D55311