libebl: Fix allocation of long strings in string table

[Petr Machata <pmachata at redhat dot com>]

Hi,

for strings of the right size just below the pagesize, not enough memory 
was allocated, and malloc internals got corrupted, eventually leading to:
$ ./src/writer ./src/dwarflint
*** glibc detected *** ./src/writer: malloc(): memory corruption: 
0x0000000013d3e860 ***

Reproducer (run through valgrind to see the invalid writes):

#include <stdio.h>
#include <string.h>
#include "../elfutils/libebl/libebl.h"
int main (void) {
   #define X 4032
   char s[X + 1] = {0};
   memset (s, '.', X);
   struct Ebl_Strtab *t = ebl_strtabinit (false);
   ebl_strtabadd (t, s, 0);
}

For long strings, the allocation was inefficient because the length of 
string was used directly, instead of rounding it up to the nearest page 
size.

Both is now fixed on the pmachata/libebl-long-strings branch.  Please 
review.

PM