use /bin/sh as the default for shell option

Mentioned in #58.

doc.go

@@ -57,7 +57,7 @@ The following options can be used to customize the behavior of lf:
 	filesep    string  (default ":")
 	ifs        string  (default "") (not exported if empty)
 	previewer  string  (default "") (not filtered if empty)
-	shell      string  (default "$SHELL")
+	shell      string  (default "/bin/sh")
 	showinfo   string  (default "none")
 	sortby     string  (default "natural")
 	timefmt    string  (default "Mon Jan _2 15:04:05 2006")

docstring.go

@@ -61,7 +61,7 @@ The following options can be used to customize the behavior of lf:
     filesep    string  (default ":")
     ifs        string  (default "") (not exported if empty)
     previewer  string  (default "") (not filtered if empty)
-    shell      string  (default "$SHELL")
+    shell      string  (default "/bin/sh")
     showinfo   string  (default "none")
     sortby     string  (default "natural")
     timefmt    string  (default "Mon Jan _2 15:04:05 2006")

etc/lfrc.example

@@ -5,12 +5,7 @@
 #set nopreview
 #set showinfo size
 
-# set shell interpreter accordingly
-# Some systems use more efficient shell implementations for non-interactive use
-# (e.g. `dash`) mapped to `/bin/sh`. By setting this option here we can tell
-# `lf` to use this implementation for shell commands. This can decrease the
-# startup times of commands but may also change the syntax that you are used to
-# (e.g. non-POSIX features provided by your regular shell).
+# interpreter for shell commands (needs to be POSIX compatible)
 #set shell /bin/sh
 
 # set internal field seperator (IFS) to ':'

main.go

@@ -15,7 +15,6 @@ var (
 	envHome   = os.Getenv("HOME")
 	envHost   = os.Getenv("HOSTNAME")
 	envPath   = os.Getenv("PATH")
-	envShell  = os.Getenv("SHELL")
 	envConfig = os.Getenv("XDG_CONFIG_HOME")
 )
 

opts.go

@@ -29,7 +29,7 @@ func init() {
 	gOpts.scrolloff = 0
 	gOpts.tabstop = 8
 	gOpts.filesep = ":"
-	gOpts.shell = envShell
+	gOpts.shell = "/bin/sh"
 	gOpts.showinfo = "none"
 	gOpts.sortby = "natural"
 	gOpts.timefmt = time.ANSIC