Fix mappings in Beeb and Prestel modes

In Beeb mode, receiving the byte on the left prints the one
on the right:

# -> #
_ -> —
` -> £

In Prestel:
# -> £
_ -> #
` -> —

Keyboard mappings adjusted so what you press is what you see.

src/conio/cterm.c

@@ -6184,6 +6184,13 @@ CIOLIBEXPORT size_t cterm_write(struct cterminal * cterm, const void *vbuf, int
 								*prnpos = 0;
 								break;
 							default:
+								// Mode 7 VDU translates to/from ASCII...
+								if (ch[0] == '#')
+									ch[0] = '_';
+								else if (ch[0] == '_')
+									ch[0] = '`';
+								else if (ch[0] == '`')
+									ch[0] = '#';
 								// "Normal" ASCII... including CR and LF in here.
 								if (buf[j] == 13 || buf[j] == 10 || (buf[j] >= 32 && buf[j] <= 127)) {
 									if (cterm->extattr & CTERM_EXTATTR_PRESTEL_MOSAIC) {

src/syncterm/CHANGES

@@ -4,6 +4,7 @@ Fix regression in Atari auto-login
 Disable Prestel ENQ/Memory in BBC Micro Mode 7
 Fix error adding a new Web List
 Enable cursor for Mode 7
+Adjust Prestel/Mode 7 mappings
 
 Version 1.5
 ------------

src/syncterm/term.c

@@ -4824,11 +4824,24 @@ doterm(struct bbslist *bbs)
 			else if (key && (cterm->emulation == CTERM_EMULATION_PRESTEL || cterm->emulation == CTERM_EMULATION_BEEB)) {
 				switch (key) {
 					case '_':
-						ch[0] = '#';
+						if (cterm->emulation == CTERM_EMULATION_PRESTEL)
+							ch[0] = '`';
+						else
+							ch[0] = '_';
 						conn_send(ch, 1, 0);
 						break;
 					case '#':
-						ch[0] = '_';
+						if (cterm->emulation == CTERM_EMULATION_PRESTEL)
+							ch[0] = '_';
+						else
+							ch[0] = '#';
+						conn_send(ch, 1, 0);
+						break;
+					case '`':
+						if (cterm->emulation == CTERM_EMULATION_PRESTEL)
+							ch[0] = '#';
+						else
+							ch[0] = '`';
 						conn_send(ch, 1, 0);
 						break;
 					case 10: