Bug 513403

SUMMARY
I was running Valgrind tests for PostgreSQL and encountered a weird behavior which I managed to reduce to the following:
When nvoking `pg_ctl` with:
```
valgrind ---trace-children=yes pg_ctl start -D ...
```
if we also supply `--suppressions` then depending on the path supplied being relative or absolute certain syscalls will or will not fail. The syscalls that do fail seem to be related to `exec(2)`. In my case both `execl` and `popen` fail with `exit code 1`.

note:
When starting postgres via pg_ctl we do need the `trace-children`: `pg_ctl` uses `exec` to start `postgres` binary which then uses `fork` to spawn multiple process, including backends that serve individual client connections. So the chain that breaks is `exec -> fork -> exec`

STEPS TO REPRODUCE
Here I build postgres from sources. The list of dependencies is relatively short. See https://wiki.postgresql.org/wiki/Compile_and_Install_from_source_code

```bash
#!/bin/env bash
# Build and initialize DB
mkdir -p /tmp/vg_repro
cd /tmp/vg_repro
git clone -b REL_18_STABLE --depth 1 --single-branch https://git.postgresql.org/git/postgresql.git
cd postgresql/
./configure --enable-cassert --enable-debug CFLAGS='-ggdb -Og -g3 -fno-omit-frame-pointer -std=c99' --prefix=/tmp/vg_repro/pgbin
make -s -j8 && make -s install
export PATH=/tmp/vg_repro/pgbin/bin:$PATH
initdb /tmp/vg_repro/pgdata --encoding=UTF8 --locale=C --no-sync
cd /tmp/vg_repro
pg_ctl -D /tmp/vg_repro/pgdata -l logfile start

# Create a table we'll use later
psql -p 5432 postgres $USER -AXqtc "create table x(a text);"

pg_ctl -D /tmp/vg_repro/pgdata stop

# The working variant with absolute path:
valgrind --leak-check=no  --suppressions=$(pwd)/postgresql/src/tools/valgrind.supp --time-stamp=yes  --trace-children=yes pg_ctl start -D /tmp/vg_repro/pgdata

# Invoke COPY FROM PROGRAM (runs popen(2))
psql -p 5432 postgres $USER -AXqtc "copy x from program '/bin/true'"
#OK

# Same thing with relative path:
valgrind --leak-check=no  --suppressions=postgresql/src/tools/valgrind.supp --time-stamp=yes  --trace-children=yes pg_ctl start -D /tmp/vg_repro/pgdata

psql -p 5432 postgres $USER -AXqtc "copy x from program '/bin/true'"
# ERROR:  program "/bin/true" failed
# DETAIL:  child process exited with exit code 1

# Don't forget to stop pg
pg_ctl -D /tmp/vg_repro/pgdata stop
```

OBSERVED RESULT
`exec`'d/`popen`'d target immediately exits with exit code 1.

EXPECTED RESULT
The target is executed normally regardless of absolute or relative paths being used in the arguments.

ADDITIONAL INFORMATION
This behavior was first spotted on CI running Ubuntu, I believe it should be present on most systems.