Add 'usergroups' state

Create `usergroups` state to collect all the groups the user running
modulecmd.tcl process is member of. Add an initStateUsergroups procedure
to initialize this state, which calls `id -G -n` to fetch these group
names. This external command is available on every Unices and its
`-G -n` arguments operate the same way.

An error will be obtained on Windows platform as `id` command is not
known there.

initStateUsergroups procedure is recorded in modulecmd.tcl as
__initStateUsergroups and renamed if initStateUsergroups procedure is
not found from Modules Tcl extension library. A more efficient version
of initStateUsergroups will be provided in the library later on (not to
depend on an external command call).

modulecmd.tcl.in

@@ -60,6 +60,7 @@ array set g_state_defs [list\
    tcl_ext_lib_loaded {0}\
    tcl_version [list [info patchlevel]]\
    term_columns {<undef> initStateTermColumns}\
+   usergroups {<undef> initStateUsergroups}\
    username {<undef> initStateUsername}\
 ]
 
@@ -352,6 +353,18 @@ proc initStateTermColumns {} {
    return [expr {![info exists cols] || $cols eq {0} ? 80 : $cols}]
 }
 
+# Get all groups of user running modulecmd.tcl process
+proc __initStateUsergroups {} {
+   # ensure groups including space in their name (found on Cygwin/MSYS
+   # platforms) are correctly set as list element
+   if {[catch {
+      return [split [string range [runCommand id -G -n -z] 0 end-1] \0]
+   } errMsg]} {
+      # fallback if '-z' option is not supported
+      return [runCommand id -G -n]
+   }
+}
+
 # Get name of user running modulecmd.tcl process
 proc __initStateUsername {} {
    return [runCommand id -u -n]
@@ -11710,6 +11723,7 @@ if {[catch {
    if {[info commands readFile] eq {}} {
       rename ::__readFile ::readFile
       rename ::__getFilesInDirectory ::getFilesInDirectory
+      rename ::__initStateUsergroups ::initStateUsergroups
       rename ::__initStateUsername ::initStateUsername
    }