tool_formparse: rewrite the headers file parser

The -F option allows users to provide a file with a set of headers for a
specific formpost section. This code used old handcrafted parsing logic
that potentially could do wrong.

Rewrite to use my_get_line() and dynbuf. Supports longer lines and
should be more solid parsing code.

Gets somewhat complicated by the (unwise) feature that allows "folding"
of header lines in the file: if a line starts with a space it should be
appended to the previous.

The previous code trimmed spurious CR characters wherever they would
occur in a line but this version does not. It does not seem like
something we want or that users would expect.

Test 646 uses this feature.
Closes #19113

tests/data/test646

@@ -42,7 +42,7 @@ It may contain any type of data.
 X-fileheader1: This is a header from a file 
 
 # This line is another comment. It precedes a folded header.
-X-fileheader2: This is 
#a
+X-fileheader2: This is #a
  folded header
 </file2>
 </client>