Mg v4.5

Changes

• Rectangle editing: rectangle-mark-mode on C-x SPC, and the
  rectangle commands on C-x r
• Registers: point-to-register, jump-to-register, copy-to-register
  and insert-register on C-x r
• New json-mode with syntax highlighting, two space indentation
• The fill column is now per buffer, like the tab width
• git-commit-mode shows the staged changes in a *diff* window next to
  the message, fills at 72 columns, and names its C-c keys in the
  echo line
• diff-mode colors the mail header of a git format-patch file, and a
  cover letter opens with its subject placeholder selected
• Filling leaves the diff in a patch or commit message alone
• conf-mode for ~/.ssh/config, ~/.Xresources and ~/.Xdefaults, and
  auto-revert-tail-mode for files under /var/log
• A buffer has one major mode: a file mode replaces the one before it,
  and an auto-execute pattern in ~/.mg always wins
• The major mode is no longer picked from the file name alone: JSON in
  a *.cfg or *.conf file opens in json-mode, and a #! line wins
  over the name

Fixes

• Fix #47: build failure with ncurses' curses.h, as on RHEL 9
• Fix build with a C library that has no wctype.h, such as uClibc-ng
  without wide-character support