move drcov2lcov file format abstraction violation parsing into drcovlib

Today the drcov file format is not formally defined and the particular strings
used are hardcoded in multiple places. This was made worse by #1829's
refactoring into the drcovlib extension.

For example, version_print() in drcovlib.c has:

```
dr_fprintf(log, "DRCOV VERSION: %d\n", DRCOV_VERSION);
dr_fprintf(log, "DRCOV FLAVOR: %s\n", DRCOV_FLAVOR);
```

With the string and precise format duplicated in read_file_header() in drcov2lcov.cpp:

```
if (dr_sscanf(buf, "DRCOV VERSION: %u\n", &version) != 1) {
```

There are similar duplicated strings in other parts of the file.

The cleanest solution would be to add new API routines to drcovlib:
- read header, return ver + flavor
- read module table: iterator?
- read bb data: iterator?