patch 8.1.2326: cannot parse a date/time string

Problem:    Cannot parse a date/time string.
Solution:   Add strptime(). (Stephen Wall, closes #)

runtime/doc/usr_41.txt

@@ -796,6 +796,7 @@ Date and Time:				*date-functions* *time-functions*
 	getftime()		get last modification time of a file
 	localtime()		get current time in seconds
 	strftime()		convert time to a string
+	strptime()		convert a date/time string to time
 	reltime()		get the current or elapsed time accurately
 	reltimestr()		convert reltime() result to a string
 	reltimefloat()		convert reltime() result to a Float