add initial implementation of 'shell' and 'shellcmdflag' options

locale/en-US/options.txt

@@ -528,13 +528,29 @@ value can be reset to half the window height with [c]:set scroll=0[c].
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-|\'nosmd'| |\'noshowmode'| |\'smd'| |\'showmode'|
-||'showmode' 'smd'|| boolean (default: on)
+|\'shell'| |\'sh'|
+||'shell' 'sh'|| string (default: on)
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 Show the current mode in the command line.
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+|\'shellcmdflag'| |\'shcf'|
+||'shellcmdflag' 'shcf'|| string (default: $SHELL or "sh", Win32: "cmd.exe")
+____
+Shell to use for executing :! and :run commands.
+____
+
+
+|\'nosmd'| |\'noshowmode'| |\'smd'| |\'showmode'|
+||'showmode' 'smd'|| boolean (default: "-c", Win32: "/c")
+____
+Flag passed to shell when executing :! and :run commands.
+
+E.g. bash -c gvim
+____
+
+
 |\'ssli'| |\'showstatuslinks'|
 ||'showstatuslinks' 'ssli'|| number (default: 1)
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