they are already specified in number of days since Epoch. Return
-2 in case of errors to support the specification of -1.
* src/usermod.c, src/useradd.c: Adapt to the new error value of
strtoday().
* src/chage.c: Remove isnum(). Adapt to the new error value of
strtoday(). Support for numerical dates is moved to strtoday().
Files with no license use the default 3-clauses BSD license. The copyright
were mostly not recorded; they were updated according to the Changelog.
"Julianne Frances Haugh and contributors" changed to "copyright holders
and contributors".
