port: add 'source' subcommand for managing sources.conf

[herbygillot]

Add port source to list, add, remove, and manage default-flag state
of entries in sources.conf.

--add supports --first (insert before existing sources), --no-sync
(append [nosync] flag), and --default (mark as the explicit default,
clearing it from other sources).

--set-default and --unset-default toggle the [default] flag on
existing entries.

--remove warns if the removed source was the only explicit default.

Bare local paths starting with / or ~ are automatically normalized and
converted to file:// URLs; local paths with spaces are supported.

Write operations are atomic (write to temp file, then rename), check
for write permission, and emit a sudo hint on failure.

Business logic uses a structured read-parse-modify-write pipeline in
macports1.0/macports.tcl: source_read_conf / source_parse_line /
source_write_conf / source_render_record handle round-tripping the
file while preserving comments and blank lines. port.tcl contains the
thin UI wrapper (action_source).

Uses file tildeexpand before file normalize for Tcl 9 compatibility,
where file normalize no longer performs tilde expansion.

Includes 20 tcltest unit tests (sources.test), a new port-source(1)
man page (port-source.1.txt), and updates to port(1) and
sources.conf(5).

Fixes: https://trac.macports.org/ticket/73683