* shared.cc (user_shared_initialize): Fetch potentially changed Cygwin

username from /etc/passwd before loading mount table.
	(shared_info::init_installation_root): New function fetching Cygwin's
	installation root dir and storing as native NT path in global shared
	memory.
	(shared_info::initialize): Call init_installation_root exactly once at
	first startup.
	* shared_info.h (SHARED_INFO_CB): Accommodate change to shared_info.
	(CURR_SHARED_MAGIC): Ditto.
	(class shared_info): Add installation_root member.
	(shared_info::init_installation_root): Declare.

	* grp.cc (pwdgrp::read_group): Call pwdgrp::load with native WCHAR path.
	* passwd.cc (pwdgrp::read_passwd): Ditto.  Avoid recursion.
	(etc::init): Take POBJECT_ATTRIBUTES instead of path_conv.
	* path.h (etc::init): Change prototype accordingly.
	* pwdgrp.h (class pwdgrp): Store path as UNICODE_STRING/PWCHAR instead
	of as path_conv.
	(pwdgrp::load): Accommodate prototype.
	* uinfo.cc (pwdgrp::load): Change argument type from char to wchar_t.
	Create native NT path here instead of calling path_conv.

	* mount.cc (find_root_from_cygwin_dll): Drop in favor of global
	initializaion in shared_info.
	(mount_info::init): Fetch native NT root dir from cygwin_shared.
	(mount_info::from_fstab): Expect native NT path and use native NT
	functions to access file.  Convert username part in user fstab path
	according to special char transformation rules.
	* path.cc (tfx_chars): Convert slash to backslash.
	(transform_chars): Implement for path given as PWCHAR.
	(transform_chars): PUNICODE_STRING version calls PWCHAR version.
	Remove useless commented code.

winsup/cygwin/path.h

@@ -350,7 +350,7 @@ class etc
   static OBJECT_ATTRIBUTES fn[MAX_ETC_FILES + 1];
   static LARGE_INTEGER last_modified[MAX_ETC_FILES + 1];
   static bool dir_changed (int);
-  static int init (int, path_conv &pc);
+  static int init (int, POBJECT_ATTRIBUTES);
   static bool file_changed (int);
   static bool test_file_change (int);
   friend class pwdgrp;