Documentation update

Rework for Doxygen >1.8. Moved large parts of the documentation to a
simplified format, making use of Markdown enhancements and fixing bad
long options.

doc_src/echo.txt

@@ -1,39 +1,41 @@
 \section echo echo - display a line of text
 
 \subsection echo-synopsis Synopsis
- <tt>echo [STRING]</tt>
+\fish{syn}
+echo [STRING]
+\endfish
 
 \subsection echo-description Description
 
-\c echo displays a string of text.
+`echo` displays a string of text.
 
 The following options are available:
 
-- \c -n, \c Do not output a newline
-- \c -s, \c Do not separate arguments with spaces
-- \c -E, \c Disable interpretation of backslash escapes (default)
-- \c -e, \c Enable interpretation of backslash escapes
-- \c -h, \c --help Display this help
+- `-n`, Do not output a newline
+- `-s`, Do not separate arguments with spaces
+- `-E`, Disable interpretation of backslash escapes (default)
+- `-e`, Enable interpretation of backslash escapes
+- `-h`, `--help` Display this help
 
 \subsection echo-escapes Escape Sequences
 
-If \c -e is used, the following sequences are recognized:
+If `-e` is used, the following sequences are recognized:
 
-- \c \\\\ \c backslash
-- \\a alert (BEL)
-- \\b backspace
-- \\c produce no further output
-- \\e escape
-- \\f form feed
-- \\n new line
-- \\r carriage return
-- \\t horizontal tab
-- \\v vertical tab
-- \\0NNN byte with octal value NNN (1 to 3 digits)
-- \\xHH byte with hexadecimal value HH (1 to 2 digits)
+- `\` backslash
+- `\a` alert (BEL)
+- `\b` backspace
+- `\c` produce no further output
+- `\e` escape
+- `\f` form feed
+- `\n` new line
+- `\r` carriage return
+- `\t` horizontal tab
+- `\v` vertical tab
+- `\0NNN` byte with octal value NNN (1 to 3 digits)
+- `\xHH` byte with hexadecimal value HH (1 to 2 digits)
 
 \subsection echo-example Example
 
-<tt>echo 'Hello World'</tt> Print hello world to stdout
+`echo 'Hello World'` Print hello world to stdout
 
-<tt>echo -e 'Top\\nBottom'</tt> Print Top and Bottom on separate lines, using an escape sequence
+`echo -e 'Top\nBottom'` Print Top and Bottom on separate lines, using an escape sequence