Use skip_spaces in more places

I looked through gdb for instance of:

      while (*p == ' ' || *p == '\t')
	p++;

... and replaced these with a call to skip_spaces.

In some cases this might slightly change the semantics, as now other
whitespace (like \r or \f) will be considered.  However I don't think
this matters.

Regression tested on x86-64 Fedora 41.

Approved-By: Simon Marchi <simon.marchi@efficios.com>