Don't limit hotkeys in user menu to simple letters only

[mooffie]

Currently, menu labels in the user menu have their 1st character interpreted as hotkeys:

@      Do something on the current file
0      Edit a bug report and send it to root
2      Call the info hypertext browser
3      Compress the current subdirectory (tar.gz)
d      Delete file if a copy exists in the other directory.
...
c      Copy to clipboard the current dir's path
C      Copy to clipboard the current file's path
D      Copy to clipboard the current file itself
Q      Copy to clipboard the first URL mentioned in the file.

This is quite limiting when there are many menu items. The attached patch lets the user specify any hotkey. Example (only the two last lines have changed):

@      Do something on the current file
0      Edit a bug report and send it to root
2      Call the info hypertext browser
3      Compress the current subdirectory (tar.gz)
d      Delete file if a copy exists in the other directory.
...
c      Copy to clipboard the current dir's path
C      Copy to clipboard the current file's path
ctrl-c Copy to clipboard the current file itself
f2     Copy to clipboard the first URL mentioned in the file.

(only the two last lines have changed: see "ctrl-c" and "f2".)

Note: because of the way hotkeys work in MC (see dlg_try_hotkey()), the hotkeys "alt-x" and "alt-X" are equivalent to pressing just "x". So alt keys shouldn't be used as hotkeys. (dlg_try_hotkey() could be modified to work around this but that's a matter for a separate ticket.)