vgdb --multi: fix various typos, indentation and such

vgdb --multi: fix various typos, indentation and such

Remove --launched-with-multi from --help-debug output since it is not
a real user option. Do add a comment in m_main.c explaining the
internal usage.

Add a top-level comment describing the three usages of vgdb.

Fix comment description of decode_hexstring, create_packet,
split_hexdecode.

Consistently use 3 space indention in send_packet and receive_packet
and next_delim_string and split_hexdecode, count_delims,
do_multi_mode.

Fix return type of count_delims to size_t.

Add a note in coregrind/m_gdbserver/server.c to sync qSupported
replies with coregrind/vgdb.c.

Use vgdb (all lowercase) and GDB (all caps) consistently in the
manual.