network/services/uhttpd: Add Basic Auth config

network/services/uhttpd: Add Basic Auth config

We add an 'httpauth' section type that contains the options:

prefix: What virtual or real URL is being protected
username: The username for the Basic Auth dialogue
password: Hashed (crypt()) or plaintext password for the Basic Auth dialogue

httpauth section names are given included as list
items to the instances to which they are to be applied.

Further any existing httpd.conf file (really whatever
is configured in the instance, but default of
/etc/httpd.conf) is appended to the per-instance httpd.conf

Signed-off-by: Daniel Dickinson <openwrt@cshore.thecshore.com>