patch 8.2.4838: checking for absolute path is not trivial

Problem:    Checking for absolute path is not trivial.
Solution:   Add isabsolutepath(). (closes #10303)

runtime/doc/usr_41.txt

@@ -904,6 +904,7 @@ System functions and manipulation of files:
 	getfperm()		get the permissions of a file
 	setfperm()		set the permissions of a file
 	getftype()		get the kind of a file
+	isabsolutepath()	check if a path is absolute
 	isdirectory()		check if a directory exists
 	getfsize()		get the size of a file
 	getcwd()		get the current working directory