[MNG-7270] Switch to shell alternative to "which"

In some circumstances the init script calls `which`, which may or may not be
available on the host system. Instead, use `command -v`, which is nearly
equivalent. One area it differs is if the command being queried is defined as
a shell alias. To avoid that, call `unset -f command` to avoid the situation
where "command" has been re-defined as a shell function.

See here for more information on this approach:
<https://pubs.opengroup.org/onlinepubs/009695399/utilities/command.html>

Tested with bash, sh (bash invoked as sh), posh, dash, zsh and mksh.

This closes #556

apache-maven/src/bin/mvn

@@ -94,7 +94,7 @@ if $mingw ; then
 fi
 
 if [ -z "$JAVA_HOME" ] ; then
-  JAVACMD=`which java`
+  JAVACMD="`\\unset -f command; \\command -v java`"
 else
   JAVACMD="$JAVA_HOME/bin/java"
 fi