Misc fixes for master

* DHCP: support requesting NTP server

  Request the NTP server from the DHCP server (option 42) and initialize
  NTP if NTP_SUPPORT is configured.

  This makes DHCP feature-equivalent to BOOTP.

* compile_magic: make httpd_mimetype_plain[] const

  Unbreak compilation by declaring httpd_mimetype_plain[] const.

* scripts/add-hardware: add vi as fallback editor

  Use the standard editor of UNIX "vi" as an additional fallback to allow
  editing the freshly generated pinning file for those folks that do
  not have $EDITOR set nor have nano installed.

* scripts/add-hardware: avoid expansions for $BUTTONS_INPUT_SUPPORT

  The quotes and backticks in the template for $BUTTONS_INPUT_SUPPORT
  make bash & dash fail, as both try to expand the text.

  Make sure the contents of this template do not get expanded by
  surrounding the delimiter with quotes.
  This keeps bash & dash happy.

* cleanup: use ' ' instead of 32 to clarify meaning

  The use of the literal ' ' for space makes the meaning much clearer
  than the use of the ASCII code.

  In the compiled code there is no difference.