head: allow '+' prefix on line count

[mknos]

• The standards document says head -n COUNT counts from one [1]
• The OpenBSD and GNU versions accept a '+' prefix on a number, so allow that here for compat

Tests:
perl head -2 head --> 2 lines
perl head -n2 head --> 2 lines
perl head -n+2 head --> 2 lines
perl head -n-2 head --> invalid

1. https://pubs.opengroup.org/onlinepubs/9799919799/utilities/head.html

[briandfoy]

changes: count from 1 and accept +N syntax