implement strftime padding specifier extensions

implement strftime padding specifier extensions

notes added by maintainer:

the '-' specifier allows default padding to be suppressed, and '_'
allows padding with spaces instead of the default (zeros).

these extensions seem to be included in several other implementations
including FreeBSD and derivatives, and Solaris. while portable
software should not depend on them, time format strings are often
exposed to the user for configurable time display. reportedly some
python programs also use and depend on them.