Add OBJF_SYMS flag and use it to decide whether or not we have already

Add OBJF_SYMS flag and use it to decide whether or not we have already
tried to read symbols from a file, for mapped objfiles.  This fixes a
memory leak (actually a constant memory growth) due to reading symbol
files with no debug info and thus generating no psymtabs or symtabs.
Most typically happened with shared libraries.