Doxyfile/Doxyfile_internal: Update to same 1.8.5 template, sync them.

This also makes the diff from Doxyfile to Doxyfile_internal more readable.

Also, put public API docs into doxy/html-api/, private stuff into
doxy/html-internal/.
Doxyfile/Doxyfile_internal: Ignore doxy/* when creating docs.
Doxygen fixes: Hide private stuff, document some structs.
hameg-hmo: Support DC LINE coupling.
appa-55ii: Don't use _t suffix for a typedef.

Names ending with _t are reserved for POSIX.

http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html
(last line in table 2)
asix-sigma: fix incorrect pointer cast (non-aligned memory and endiannes issue)

This fixes the following warning:

asix-sigma.c: In function 'receive_data':
asix-sigma.c:1064:4: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
    devc->state.lastts = *(uint16_t *) buf - 1;
    ^
rigol-ds: Rename and document some confusingly named variables.
rigol-ds: Use rigol_ds_channel_start() function for legacy protocol too.
rigol-ds: Unify code for counting expected incoming bytes.
rigol-ds: Unify partial read handling.
rigol-ds: Select channels before issuing RUN command.
rigol-ds: Use set_cfg wrapper for capture setup commands.
rigol-ds: Use correct analog frame size for VS5000 series.
rigol-ds: Fix rigol_ds_channel_start() for digital channels.
Add config keys for upcoming RF demodulator drivers.

Langford driver will be first.