Search for uiserver more agressively

This is supposed to work for both gpg4win 2.x and 3.x and the new
install layout.

• configure.ac (gpg4win-3-reg-keys): Remove option.
• src/common.c (default_homedir): Fallback to Gpg4win 2 regkey. (get_gpg4win_dir): New. Search at multiple places. (get_data_dir): Use get_gpg4win_dir.
• src/common.h (get_gpg4win_dir): Declare. (GPG4WIN_REGKEY_2, GPG4WIN_REGKEY_3): New for old and ney keys.
• src/engine-assuan.c (get_uiserver_name): Rewrite to look for more names.
• src/main.c (get_locale_dir): Use get_gpg4win_dir.

This fixes gpgol for the new install layout where the uiserver
executables are installed in the bin subdirectory.