patch 8.1.1510: a plugin cannot easily expand a command like done internally

Problem:    A plugin cannot easily expand a command like done internally.
Solution:   Add the expandcmd() function. (Yegappan Lakshmanan, closes #4514)

runtime/doc/eval.txt

@@ -2326,6 +2326,7 @@ extend({expr1}, {expr2} [, {expr3}])
 exp({expr})			Float	exponential of {expr}
 expand({expr} [, {nosuf} [, {list}]])
 				any	expand special keywords in {expr}
+expandcmd({expr})		String	expand {expr} like with `:edit`
 feedkeys({string} [, {mode}])	Number	add key sequence to typeahead buffer
 filereadable({file})		Number	|TRUE| if {file} is a readable file
 filewritable({file})		Number	|TRUE| if {file} is a writable file
@@ -4218,6 +4219,14 @@ expand({expr} [, {nosuf} [, {list}]])				*expand()*
 		See |glob()| for finding existing files.  See |system()| for
 		getting the raw output of an external command.
 
+expandcmd({expr})					*expandcmd()*
+		Expand special items in {expr} like what is done for an Ex
+		command such as `:edit`.  This expands special keywords, like
+		with |expand()|, and environment variables, anywhere in
+		{expr}.  Returns the expanded string.
+		Example: >
+			:echo expandcmd('make %<.o')
+<
 extend({expr1}, {expr2} [, {expr3}])			*extend()*
 		{expr1} and {expr2} must be both |Lists| or both
 		|Dictionaries|.

runtime/doc/usr_41.txt

@@ -1,4 +1,4 @@
-*usr_41.txt*	For Vim version 8.1.  Last change: 2019 May 29
+*usr_41.txt*	For Vim version 8.1.  Last change: 2019 Jun 09
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
@@ -609,6 +609,7 @@ String manipulation:					*string-functions*
 	strcharpart()		get part of a string using char index
 	strgetchar()		get character from a string using char index
 	expand()		expand special keywords
+	expandcmd()		expand a command like done for `:edit`
 	iconv()			convert text from one encoding to another
 	byteidx()		byte index of a character in a string
 	byteidxcomp()		like byteidx() but count composing characters