* cygheap.cc (init_cheap): Move username initialization.

* cygheap.cc (init_cheap): Move username initialization.
(cygheap_init): Here.
* shared_info.h (mount_info): Add a sys_mount_table_counter field.
(shared_info): Ditto.
* path.cc (mount_info::conv_to_win32_path): Check that our mount table is in
sync with the system mount table and reinitialize it if not.
(mount_info::add_reg_mount): Bump sys_mount_table counters if the system mount
table changes.
(mount_info::del_reg_mount): Ditto.
(mount_info::write_cygdrive_info_to_registry): Ditto.
(mount_info::remove_cygdrive_info_from_registry): Ditto.