Add 'username' state

Create `username` state to collect the name of the user running
modulecmd.tcl process. Add an initStateUsername procedure to initialize
this state, which calls `id -u -n` to fetch username. This external
command is available on every Unices and its `-u -n` arguments operate
the same way.

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

initStateUsername procedure is recorded in modulecmd.tcl as
__initStateUsername and renamed if initStateUsername procedure is not
found from Modules Tcl extension library. A more efficient version of
initStateUsername 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}\
+   username {<undef> initStateUsername}\
 ]
 
 # Configuration option properties (superseding environment variable, default
@@ -351,6 +352,11 @@ proc initStateTermColumns {} {
    return [expr {![info exists cols] || $cols eq {0} ? 80 : $cols}]
 }
 
+# Get name of user running modulecmd.tcl process
+proc __initStateUsername {} {
+   return [runCommand id -u -n]
+}
+
 # Initialize Select Graphic Rendition table
 proc initConfColors {envvar value validvallist intvallist} {
    # overriden value coming from environment
@@ -11697,6 +11703,7 @@ if {[catch {
    if {[info commands readFile] eq {}} {
       rename ::__readFile ::readFile
       rename ::__getFilesInDirectory ::getFilesInDirectory
+      rename ::__initStateUsername ::initStateUsername
    }
 
    # source site configuration script if any